1. Field of the Invention
The present invention relates to a semiconductor process, and more particularly to a high density plasma chemical vapor deposition process and a method for improving uniformity of thickness of a thin film.
2. Description of the Related Art
In semiconductor manufacturing process, chemical vapor deposition (CVD) is a widely used technique for forming thin films. It usually includes atmospheric CVD (APCVD), low pressure CVD (LPCVD), plasma enhanced CVD (PECVD) and high density plasma CVD (HDPCVD). The film formed by HDPCVD is denser, more capable of preventing moisture penetration and more uniform than those formed by the other techniques. With the advancement of semiconductor technology, devices in integrated circuits becomes more compact. The gap-fill ability of HDPCVD for devices is better than those of the other techniques. Accordingly, HDPCVD has become a core technique for sub-micron semiconductor technology.
HDPCVD process should be performed at the deposition systems in the reaction chambers. During a thin film deposition, reaction gases are released from gas output holes and diffuse to the wafer for forming a thin film thereon. For example, a deposition system manufactured by Novellus, Corp. comprises eight gas output holes. These gas output holes are allocated in the chamber around the wafer. During the thin film deposition, reaction gases are released from these eight gas output holes for forming the thin film on the wafer.
Due to different distances from these gas output holes to different areas of the wafer, the thickness of the thin film formed by the deposition system manufactured by Novellus Corp. is not uniform at the edge of the wafer, i.e. near to these gas output holes. The non-uniformity of thickness results in wave patterns at the edge of the wafer as shown in FIG. 1. The reason of causing the wave patterns is that the reaction gases cannot uniformly diffuse to different areas of the wafer and the non-uniformity leads to different accumulations of the reaction gases on different areas of the wafer. Accordingly, a thinner film is formed along the direction 100 of the gas output holes as shown in FIG. 1. A thicker film, however, is formed on the other areas 102 as shown in FIG. 1. The wave-pattern issue becomes worsen at the region closer to these gas output holes. The wave-pattern issue causes non-uniformity of thickness of the thin film and affects the characteristics of the thin film.